Un-Subdivide - Remove loops on one axis
Method 1:
First you would want to separate the zig-zag part from the bag, select the edge using Alt-Click that you want to cut like in the picture.

Then you can press V and move your mouse upward to rip the edge loop off.

Then you can select the vertical edge loops using Alt-Click in a pattern: select 3, not select 1 (It doesn't have to be 3 and 1 but I found it to work quite well). Unfortunately, I couldn't think off an automatic way to do this, so you may have to do this manually.

After you select all the edge loops, click X and select Dissolve edges.

This is the final outcome


Method 2: This is faster but it may changes the mesh a little.
Use Vertex>Smooth vertices and change the settings until you are satisfy with the result.